Transmissible murine colonic hyperplasia is a newly discovered disease of mice which has marked potential as an animal model system to study preneoplastic lesions of the colon. Our specific aim is to define this system, and then attempt to manipulate it in order to examine possible methods of inducing it to progress to neoplasia. Major emphasis will be placed on 1) isolating and characterizing the apparent agent, 2) studying the pathogenesis with specific attention given to host range and genetic susceptibility, 3) cell kinetics, 4) ultrastructural changes, 5) immunological host responses, and 6) methods to induce it to progress to neoplasia.